According to the Power over Ethernet (PoE) Institute of Electrical and Electronics Engineers (IEEE) Standard 802.3af/at a powered device (PD) is powered by power sourcing equipment (PSE) via an Ethernet cable. The PSE is, for instance, a switch and the PD are, for example, internet protocol (IP) cameras, IP phones, wireless access points, luminaires, sensors, fans, et cetera. In accordance with the standard, the PSE is able to determine whether a cable is disconnected, connected to a non-PoE device, or connected to a PoE-compliant PD. This is accomplished in three phases (which follow the Idle state): Detection phase, Classification Phase, and 2-event Classification; after which the PD is powered (in the Operational state).
In the detection phase, the PSE detects whether a PD is connected. During the Detection phase, a PoE-compliant PD will provide a valid detection signature using a capacitor (50 nF-120 nF) and a resistor (25 kΩ; a present only during the Detection phase). As an example, the PSE can apply two voltages in the range of 2.8V to 10V and measures corresponding currents to determine the presence of the signature resistor. Other methods are also allowed, such as sourcing current. In the classification phase, PSE and PD are distinguished as Type-1 complying with the IEEE 802.3af power levels (12.95 W), or Type-2 complying with the IEEE 802.3at power levels (25.5 W). Additionally the 802.3at standard provides a different method of determining the power classification. A Type-2 PSE has the option of acquiring PD power classification by performing 2-event classification (Layer 1) or by communicating with the PD (Layer 2). At the same time a Type-2 PD must be able to identify a Type-2 PSE and communicate over Layer 1 and 2. As part of the 2-event classification, the PSE provides a fixed voltage between 15.5V and 20.5V to the PD.
After determining a PD is connected (and the optional power classification) the PSE powers the PD until the PD no longer uses power. The standard specifies that if a PD requires little to no power, such as in a standby mode, the PD must generate a Maintain Power Signature (MPS). According to IEEE802.3at, the MPS consists of two components, an AC MPS component and a DC MPS component. The PSE shall monitor the DC MPS component, the AC MPS component, or both. The AC MPS component is present when an AC impedance is detected at the power interface equal to or lower than 27 kΩ. The DC MPS requires the PD to draw at least 10 mA, lasting a minimum period of 60 ms. The minimum dropout period is 300 ms (may not drop out) and the maximum drop out period is 400 ms (must drop out).
If no MPS is detected by the PSE, the PSE may disconnect power to the PD. This prevents the PSE from providing power to a port that has been disconnected, which can in turn prevent a person touching such a port (or a cable connected to that port) from getting an electric shock. As another example, this can prevent damage to a non-PoE device being connected to a port of the PSE to which power is applied.
Currently discussions are coming up to use the PoE standard for all kinds of loads like lighting equipment (sensors, switches, light sources etc.) or entertainment appliances like active speakers, internet radios, DVD player, set-top boxes and even TV sets. A future standard is then needed that support higher power levels, such as levels up to 60 W or more per Cat5/6 connection. Such discussions are not only ongoing related to PoE, but also related to other (similar) standards such as the EMerge Alliance Occupied Space Standard or the USB Power Delivery standard. Together with this development of higher power devices being powered through such power distribution systems, there is also a development that the number of devices that is powered through these systems increases. There is therefore a need to develop a power distribution system with increased electrical efficiency.